1. Field of the Invention
This invention relates to a method of manufacturing an information recording medium, and in particular to a method of manufacturing an information recording medium using an optical disk.
2. Description of the Prior Art
The principle of recording information on a thin film (recording film) by irradiating with laser light is well-known. Techniques are also known which use an atomic arrangement change due to irradiation with laser light, such as a phase change or phase transformation of a film material. As this phase-change is accompanied by very little deformation of the thin film, it has the advantage that two disks can be directly glued together to make an information recording medium having a double-sided disk structure.
Usually, these information recording media comprise for example a protective layer, a recording layer such as GeSbTe, a protective layer and a reflective layer on a substrate.
However, in rewritable phase-change optical disks, such as DVD-RAM, because the protective layer penetrates into the recording film, crystallization is poor and the optimal crystallization rate is not realized, or, crystallization occurs too early and the amorphous state transformation is inadequate, so the reflectivity level after overwrite fluctuates. Hence, an interface layer having a good thermal stability was provided between the protective layer and the recording film to prevent the penetration of the protective layer into the recording film. For example, an interface layer of oxide or nitride was provided which contacts the recording film (Prevention of protective layer counter-diffusion between recording films in phase-change optical disks by oxide interface layer: Yasushi Miyauchi, Motoyasu Terao, Akemi Hirotsune, Makoto Miyamoto, Nobuhiro Tokuyado: Japan Society of Applied Physics Lectures, Vol. 3, p. 29-ZK-12 (Spring, 1998), 1127). Compared to the case of a double-sided (ZnS)80xe2x80x94(SiO2)20 protective layer, the crystal nucleus growth rate and crystal growth rate are higher, and the crystallization rate is rapid. Moreover, by using a nitride on both interface layers, diffusion of the recording film material is suppressed and the crystallization rate is optimized. (Phase-change optical disks with nitrides on both sides of the recording film, Mayumi Otoba, Noboru Yamada, Hiroyuki Ota, Katsumi Kawahara, Japan Society of Applied Physics Lectures, p. 29-ZK-13 (Spring, 1998) 1128) and N. Yamada, M. Otoba, K. Kawahara, N. Miyagawa, H. Ota, N. Akahira and T. Matsunaga: Phase-change optical disk having a nitride interface layer: Jpn. J. Appl. Phys. Part 1, 37 (1998) 2104. To achieve many overwrites, diffusion into the recording film, such as diffusion of Zn, S from the upper and lower ZnSxe2x80x94SiO2 protective layers into the recording film, must be prevented. For this purpose, it is effective to provide the interface layer.
In JP Hei 5-144083, the interface layer is provided on the upper and lower sides of a recording film, using TaO, CrO and MnO, for example, as the interface layer. Further, in JP Hei 6-124481, JP Hei 10-21582 and JP Hei 8-287516, TaO and other compounds (e.g., ZnS, TaS) are provided as a layer in contact with the recording film.
In JP Hei 5-144083, for example, the thickness of the interface layer is 3 nm, 150 nm, the thickness of the first protective layer is 150 nm, and (thickness of interface layer)/(thickness of interface layer+thickness of first protective layer) is 3/(150+3)=0.02.
In this specification, the term xe2x80x9cphase-changexe2x80x9d is used to describe not only a phase change between crystal and amorphous states, but also in the sense of a phase change such as fusion (change to liquid phase) and recrystallization, and a phase change between crystalline states. Mark edge recording means a recording method wherein the edge of a recording signal is made to correspond to xe2x80x9c1xe2x80x9d, and parts between marks and the inside of a mark are made to correspond to xe2x80x9c0xe2x80x9d. In this specification, optical disk means a disk on which information is recorded by irradiation of light, and/or a disk wherein information can be reproduced by irradiation of light.
However, in an interface layer material having good heat stability, the sputter rate is very small, and this slowed down overall production. Also, information recording media using interface layer materials of good heat stability suffered from the drawback that they were not suitable for mass production.
It is therefore an object of this invention to provide an information recording medium having good thermal stability which, due to the use of an interface layer with a high sputter rate, also has good recording/reproduction characteristics and is excellent for mass production, and to provide a method of manufacturing same.
To resolve the above problems, this invention provides an information recording medium having the following characteristics. Specifically, a Taoxe2x80x94O interface layer with good thermal stability and high sputter rate is used. In this way, it is possible to provide an information recording medium which has good recording/reproduction characteristics, and is excellent for mass production. In the past, materials used for the interface layer either had a low sputter rate or their thermal stability was poor, and they could not satisfy both requirements. This invention reconciles these objectives.
The interface layer is provided between a first protective layer and a recording film, which is in contact with a recording layer. The thickness of the interface layer is 0.20 or more but 0.67 or less of the total of the first protective layer and interface layer, and the composition of the interface layer contains tantalum (Ta) and oxygen (O).
Hence, rewritability is good, and the productivity is improved because the sputter rate of the interface layer is high. Herein, rewritability was determined by examining the recording waveform deterioration due to overwrites described later, or more specifically the reflectivity of the crystal level (Ic).
By arranging the thickness of the interface layer to be from 0.20 or more but 0.67 or less of the total thickness of the first protective layer and interface layer as described above, productivity is improved by approximately 170% compared to the prior art and this already gives a satisfactory improvement, but if the thickness of the interface layer is arranged to be from 0.30 or more but 0.60 or less of the total thickness of the first protective layer and interface layer, productivity is further improved and attains approximately 195% or more compared to the prior art which is very desirable.
Regarding the recording waveform deterioration due to overwrite, we examined the reflectivity variation of the crystal level (Ic).
Hereafter, the method of evaluating the reflectivity variation is described. We shall give details only for evaluating the reflectivity variation for groups, but an identical method may be used for lands.
First, the disk to be measured is set in a tester, and rotated. An optical head is then brought into the vicinity of the track to be measured. Autofocus is applied at this position, and the tracking error signal (difference signal) is monitored on an oscilloscope. The autofocus gain is controlled so that the tracking error signal amplitude in the group is maximized (AF offset control). Next, tracking is applied to the group while the autofocus is still applied. Recording is then performed by varying the laser power with a random signal. The recording power is found at which the difference (asymmetry) between the centerline of the envelope of the signal corresponding to the 3T (shortest) mark and space, and the centerline of the envelope of the signal corresponding to the longest mark and space, is +5%, and this is taken as the optimum recording power. Next, the relation between the radial (radial direction)-tilt and the jitter value after 10 overwrites (optimum power) is measured by a time interval analyzer (TIA), and the radial-tilt at which jitter is minimized, is calculated. In other words, jitter is measured while the radial-tilt is varied, the radial-tilt is found at which jitter is minimized, and this is taken as the optimum radial-tilt. Next, tracking offset control is performed. First, 10 overwrites are performed at the optimum power on the lands on both sides of the group. Then, the crosstalk from the lands in the group is measured by a spectral analyzer. The tracking gained is adjusted so that this crosstalk is minimized. Preferably, the optimum radial-tilt is found again, and tracking offset control is repeated. Finally, after the AF offset control, tracking offset control and radial-tilt control are completed for the group, and the beam is moved to the track for measuring reflectivity variation. The reproduce signal (sum signal) of the ID part (part representing the address information in the pit) situated one half a track away to the left and right of this track, is monitored, and after one long mark is recorded, the reflectivity, i.e., the voltages of the crystal level (Ic) and the amorphous level (Ia) are measured. Further, the reflectivity after 10 to 10000 overwrites is measured.
In the same way, the reflectivity in a land is measured. Herein, regarding the relation between crystal level and reflectivity, when Ic is 85 mV, the reflectivity of the medium is 15%, so it is preferred that Ic is 85 mV. If the reflectivity of the medium is less than 15%, the degree of modulation of the record/reproduce signal is low, AF or tracking become unstable, and neither recording nor reproduce can be performed, therefore it is preferred that the reflectivity is 15% or higher. Due to these reasons, even in the DVD-RAM specification, the reflectivity is determined to be 15% or higher.
Next, another aspect of this invention will be described. In the other aspect of this invention, the following composition is proposed. Specifically, the interface layer is provided between the first protective layer and recording film, this interface layer is in contact with the recording film, the interface layer contains tantalum (Ta), oxygen (O) and a metal element (any of M=Sb, Bi, Al, Ga, In, Si, Ge, Sn, Pb, Zn, Cu, Ag, Ni, Pd, Pt, Co, Rh, Ir, Fe, Ru, Os, Mn, Re, Cr, Mo, W, V, Nb, Ti, Zr, Hf, Sc, Y and La), and the Ta amount in the interface layer (atomic %) is in the range of 3 atomic % to 28 atomic %. As the reflectivity does not change even if 10000 overwrites or more are performed, the medium is stable to multiple overwrites.
The interface layer may also contain sulfur in addition to tantalum, oxygen and the above-mentioned metallic elements. That is, the interface layer contains tantalum (Ta), oxygen (O), sulfur (S) and a metallic element (any of M=Bi, Ga, In, Sn, Pb, Zn, Ag, Ni, Co, Fe, Mn, Mo, W, Nb, Zr and Hf), and the Ta amount in the interface layer (atomic %) is in the range of 3 atomic % to 28 atomic %. Thus, as the reflectivity does not change even if multiple overwrites are performed, multiple overwrites can be performed with high stability.
Another aspect of this invention will now be described. In this aspect, the interface layer is provided between the first protective layer and recording film, the interface layer is in contact with the recording film, the interface layer contains tantalum (Ta) and oxygen (O), and the absolute value of the extinction coefficient k of the interface layer is 0.22 or less. In this way, it is possible to satisfy the reflectivity standard of 15% or more for DVD-RAM.
This invention is particularly effective for recording densities (track pitch, bit pitch) higher than 4.7 GB, which is the specification for DVD-RAM. When the wavelength of the light source is not in the vicinity of 660 nm or the aperture (NA) of the collimating lens is not 0.6, the invention has an effect at recording densities converted from these by wavelength ratio or NA ratio for both the radial direction and circumferential direction.
The basic technology of recording devices (optical disk drives) using the phase-change recording medium of this invention is as follows.
1 Beam Overwrite
A phase-change recording medium is normally rewritten by overwrites (rewriting information by overwriting without first erasing). This principle is shown in FIG. 7. If the recording medium is melted at a high laser power, it is rapidly cooled after irradiation, and the recording mark will be in the amorphous state whether the previous state is crystalline or amorphous. If it is heated to a temperature with a high crystallization rate below the melting point at an intermediate laser power, parts which were previously in an amorphous state become crystalline. Parts which were originally crystalline remain crystalline. With DVD-RAM, moving images are often recorded, and it may be expected that long periods of information will be recorded on one occasion. In this case, if recording is performed after first erasing all previous data, twice as long is required and a very large buffer memory may be necessary. Therefore, it is absolutely necessary that overwrites can be performed.
Mark Edge Recording
With DVD-RAM and DVD-RW, mark edge recording which permits a high density recording is used. In mark edge recording, the two edges of a recording mark formed on the recording film are made to correspond to the 1 of digital data. Consequently, a high recording density can also be achieved by making the shortest recording mark correspond not to one, but to two or three reference clocks. In DVD-RAM, 8-16 modulation is adopted in which the mark is made to correspond to three reference clocks. As shown by the comparison of FIG. 8, as compared to mark position recording in which the center position of a circular recording mark is made to correspond to the 1 of digital data, high density recording can be performed without making the recording mark very small. However, a recording medium is required wherein the shape distortion of the recording mark is very small.
Format
As shown by the position of the header zone at the beginning of each sector in FIG. 9, DVD-RAM are formatted by dividing one circumference into 24 sectors so random access recording can be performed. Therefore, a wide range of devices such as DVD video cameras and DVD video recorders can be used from the internal storage device of a personal computer.
Land Groove Recording
In DVD-RAM, crosstalk is reduced by land-groove recording wherein recording is performed both in the tracking groove and in the projections between grooves. In land-groove recording, the phenomenon is used that when the groove depth is in the vicinity of xcex/6n (xcex is the laser wavelength and n is the refractive index of the substrate) relative to the recording mark for contrast (light/dark), the recording mark of the adjacent track becomes difficult to see in both lands and grooves, hence in the example of a 4.7 GB DVD-RAM, the track pitch is as narrow as 0.615 xcexcm. Due to the phase difference between recording marks and other parts, i.e. the phase difference component of the reproduce signal, crosstalk tends to be generated, so a design is required which sufficiently suppresses this. The phase difference component of the reproduce signal is added as a reverse phase to the light/dark reproduce signal of the lands and grooves, and also causes an imbalance in the reproduce signal level of the lands and grooves.
ZCLV Recording Method
In a phase-change recording medium, if the recording wavelength is not changed, it is desirable to record at an optimum linear velocity corresponding to a crystal growth rate which allows good recording/reproduction characteristics to be obtained. However, when areas between recording tracks with different radii on the disk are accessed, it takes time to change the rotation speed in order to make the linear velocity the same. Hence, in DVD-RAM, as shown in FIG. 11, the ZCLV (Zoned Constant Linear Velocity) method is used wherein the radial direction of the disk is divided into 24 zones so that the access speed does not decrease. The rotation speed in one zone is constant, and the rotation speed is changed only when it is required to access another zone. In this method, the linear velocity is slightly different between the innermost track and outermost track of the zone, so the recording density is also slightly different, but recording can be performed at effectively the maximum density over the whole disk. The technique used with recording medium of this invention is described below.
Absorption Control
With a high linear velocity (8.2 m/s) medium such as a 4.7 GB/side medium, pre-erase (where the recording mark is first erased in a belt-shaped area in the temperature range 300xc2x0 C. to 550xc2x0 C. before the area in which the recording film is melted by the light spot irradiation), which can be expected in a lower linear velocity recording medium such as a 2.6 GB/side DVD-RAM (6 m/s), cannot be expected to occur sufficiently. Therefore, the absorption ratio Ac/Aa inside and outside the recording mark must be maintained at 0.8 or more. By performing absorption control, the mark edge position can be precisely recorded. In absorption control, a method exists wherein the reflective layer is made thinner so that, in a recording mark which has a low reflectivity, the light is transmitted and optical absorption in the recording film does not become excessive (Noboru Yamada, Nobuo Akahira, Kenichi Nishiuchi, Keisho Furukawa: A high-speed overwrite phase change optical disk, Japan Society of Electronic and Information Engineers, Technical Research Report MR92-71, CPM92-148 (1992), 37). The reflective layer uses Cr or Al, or an alloy containing one of these, for the purpose of absorption ratio control and maintaining high contrast. This layer absorbs light and transmits light to a suitable degrees hence light which has been transmitted through the recording film in the recording mark which has a low reflectivity, is reflected by the reflective layer and reabsorbed by the recording film. Thus, the temperature does not rise too much, and Ac/Aa is controlled to be 1 or more.
In high density phase change optical disks, as the track pitch is narrow, it is necessary to consider the phenomenon known as cross-erase wherein part of the recording mark which is already written on the adjacent track is erased. To prevent this cross-erase, transverse diffusion of heat is important. One reason for this is that heat is not easily transmitted to the adjacent tracks in transverse diffusion. Also, if Ac/Aa is larger than 1, the temperature rise of the recording marks of the adjacent tracks is less, and this also works to prevent cross-erase.
To prevent cross-erase, it is also important to prevent recrystallization. This is because, as shown in FIG. 12, in recrystallization which occurs from the periphery after the melting of the recording film during recording, the part which remains as an amorphous recording mark becomes narrower, so it is necessary to melt a wider area to form a recording mark of a predetermined size and the temperature of adjacent tracks tends to rise. If the heat is diffused in a transverse direction, recrystallization can also be prevented. This is because when the recording mark is formed, the heat in the center part diffuses in a lateral direction so that cooling around the melting area is delayed, thus the tendency to recrystallization is prevented.
First Protective Layer
The first protective layer is a laminated film provided on the light incidence side of the recording film to protect the recording film. As regards the refractive index and film thickness, to maintain high optical contrast, it is preferred that in the vicinity of a reproduce wavelength of 660 nm, the refractive index n is from 1.5 to 2.3, and the film thickness is from 100 to 150 nm. To obtain good recording sensitivity, it is preferred that the heat conductance on this layer is at least approximately one digit higher than that of the recording film. Even outside these limits, the effect of the interface layer of this embodiment was still observed.
Recording Waveform
The following relation exists between the recording waveform and recording mark shape. For example, in a 4.7 GB DVD-RAM, the shortest mark length is 0.42 xcexcm and the linear velocity is 8.2 m/s. Due to this, a recording pulse which forms one recording mark is divided into plural pulses. To precisely form the recording mark, more emphasis is placed on correct heating than on preventing build-up of heat, and as shown in FIG. 13, the recording waveform has few or no parts with a lower erase power level. Also, as already stated, it is necessary to perform adaptive control of the width of the first pulse and last pulse which form the recording mark (adaptive control: adjustment of position where last pulse which forms the previous mark finishes and first pulse which forms the next mark starts according to length of space involved and length of previous mark).
The high-performance techniques may be summarized as follows:
1. Techniques contributing to narrow track pitch Land-groove recording, absorption control
2. Techniques contributing to narrow bit pitch Mark edge recording, ZCLV recording method, absorption control, interface layer, adaptive control recording waveform
3. Techniques contributing to high speed 1 beam overwrite, recording film composition, absorption control, interface layer
4. Techniques contributing to high numbers of overwrites.
Interface Layer
One layer has plural roles as described above, and the functions of each layer are inter-related in a complex manner. The interface layer contributes to narrow pitch, high speed and high numbers of overwrites. Therefore, it is important to choose the optimum combination and thicknesses of laminated films to achieve high performance.